It is well known that cis-1,4-polybutadiene can be prepared by polymerizing 1,3-butadiene monomer with nickel based catalyst systems. Such nickel based catalyst systems contain (a) an organonickel compound, (b) an organoaluminum compound, and (c) a fluorine containing compound. Such nickel based catalyst systems and their use in the synthesis of cis-1,4-polybutadiene is described in detail in U.S. Pat. Nos. 3,856,764, 3,910,869, and 3,962,375.
The cis-1,4-polybutadiene prepared utilizing such nickel based catalyst systems typically has a high molecular weight. Due to this high molecular weight, the cis-1,4-polybutadiene is generally oil extended. However, this precludes the cis-1,4-polybutadiene from being utilized in many applications. For instance, such oil extended rubbers cannot be utilized in tire sidewalls which contain white sidewall compounds. In any case, there is a large demand for cis-1,4-polybutadiene having a reduced molecular weight which can be processed without being oil extended.
Various compounds have been found to act as molecular weight reducing agents when used in conjunction with the nickel based catalyst system. For instance, U.S. Pat. No. 4,383,097 discloses that alpha-olefins, such as ethylene and propylene, act as molecular weight reducing agents when utilized in conjunction with such three component nickel catalyst systems. Canadian Patent 1,236,648 indicates that 1-butene, isobutylene, cis-2-butene, trans-2-butene, and allene act as molecular weight regulators when used in conjunction with such nickel based catalyst systems. U.S. Pat. No. 4,383,097 reveals that certain nonconjugated diolefins, such as 1,4-pentadiene, 1,6-heptadiene, and 1,5-hexadiene, act as molecular weight reducing agents when utilized in conjunction with such catalyst systems. U.S. Pat. No. 5,100,982 indicates that cis-1,4-polybutadiene having reduced molecular weight and a broad molecular weight distribution can be synthesized with certain nickel based catalyst systems in the presence of halogenated phenols, such as para-chlorophenol.
The processability of cis-1,4-polybutadiene rubbers can be improved by simply lowering their molecular weight. However, this approach also typically leads to increased cold flow. Accordingly, the use of conventional molecular weight reducing agents, such as α-olefins, to improve rubber processability leads to compromised cold flow characteristics.
U.S. Pat. No. 5,451,646 discloses that para-styrenated diphenylamine acts as molecular weight reducing agent when employed in conjunction with nickel based catalyst systems which contain (a) an organonickel compound, (b) an organoaluminum compound, and (c) a fluorine containing compound. U.S. Pat. No. 5,451,646 indicates that para-styrenated diphenylamine also acts to improve the processability of cis-1,4-polybutadiene rubbers prepared in its presence utilizing such nickel based catalyst systems. In other words, para-styrenated diphenylamine can be employed in conjunction with such nickel based catalyst systems to reduce the molecular weight of the rubber without sacrificing cold flow characteristics. The para-alkylated diphenylamine which remains in the rubber produced also acts in a manner which provides it with antioxidant protection. The para-styrenated diphenylamine accomplishes three major objectives. It reduces the molecular weight of the polymer, regulates polymer macrostructure and acts as an antidegradant.
U.S. Pat. No. 5,451,646 more specifically discloses a process for producing cis-1,4-polybutadiene having reduced molecular weight and improved processability which comprises polymerizing 1,3-butadiene in the presence of (a) an organonickel compound, (b) an organoaluminum compound, (c) a fluorine containing compound, and (d) para-styrenated diphenylamine; wherein the organoaluminum compound and the fluorine containing compound are brought together in the presence of the para-styrenated diphenylamine. The successful modification of catalyst activity in this case is dependent on the reaction between the para-styrenated diphenylamine and the fluorine containing compound. The complex formed when these two components come into contact is insoluble in aliphatic solvents, such as hexane. This insolubility in aliphatic solvents necessitates the use of undesired aromatic solvents in the production process to help solubilize the complex. However, even with the use of aromatic solvents the material is sensitive to precipitation and line fouling when exposed to mixed streams containing aliphatic solvents.